Sedimentation units, aggregating suspensions

Figure 4. Observed removal efficiencies of specifically aggregated suspensions in varied separation units (sedimentation tanks).

Such deviation from the simple concept of sedimentation — the analogy holds for flotation as well with only the direction of motion changed — can be explained by the interaction of aggregate properties with the flow structure in the separation unit if there are regions with higher shear stress in a separation apparatus then the more stable floe will be separated more effectively than the shear-endangered one. Or, similarly, the less heterodisperse suspension will be separated more effectively in sedimentation tanks than suspensions with a broader spectrum of aggregate sizes. 

Figure 1. Effects of the concentration of dextrans with various molecular weights on three indices of BBC aggregation (16). MAI indicates the average number of RBCs in each aggregation unit counted under the microscope. ESR is the maximum rate of sedimentation of erythrocytes in a calibrated tube, with corrections made for changes in viscosity and density of the suspending medium following the addition of dextrans. The relative viscosity (t)r) is the ratio of the viscosity of RBC suspension to that of the suspending medium at a shear rate fo 0.1 sec 1. The RBC concentration of the suspension was 1% for MAI and 45% for ESR and r)r measurements. The vertical bars represent SEM. (A), Dx 40 (O), Dx 80 (M), Dx 150 (A), Dx 500 ( ), Dx 2000.

The separation units performance was analyzed by introducing a delta impulse. Over a period of less than 1 min the suspension from the aggregation chamber was directed into the sedimentation or flotation unit by means of a three-way switch. The flow pattern in the separation reactor was as little disturbed as possible. The resulting small number of aggregates was then fol-... 

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